At the construction site of developed land, road, dam and the like, soil cutting and soil filling are performed to form a slope surface. When such slope surface is left as it is, the slope surface gets eroded by rain and weathering, thereby causing accidents such as landslide and rock fall. Accordingly, in order to prevent erosion of soil, Patent Literature 1 discloses formulating drying inhibitor for soil comprising water-swelling and water-absorbing resin, surfactant, and synthetic resin emulsion into a spraying material by a ratio of 0.5 to 1.5 kg per 1 m3 of the spraying material, and spraying such spraying material onto the slope surface.